1. Field of the Invention
The present invention relates to an insulation composition for a DC power cable and the DC power cable prepared using the same. Specifically, the present invention relates to an improved insulation composition for a DC power cable, which can prevent decrease of DC dielectric strength and decrease of impulse strength caused by accumulation of space charges, and the DC power cable prepared using the same.
2. Background of the Related Art
A cross-sectional structure of a general power cable is as shown in FIGS. 1a and 1b. As shown in FIGS. 1a and 1b, the power cable may include a conductor 1 at the center, and an inner semi-conductive layer 2, an insulation layer 3, an outer semi-conductive layer 4, a sheath layer 5 and the like, which are formed around the conductor 1.
When a high DC voltage is applied to the power cable, electric charges are injected into the inner semi-conductive layer 2, the insulation layer 3 and the like from the conductor 1, and space charges are formed in the insulation layer 3 affected by injection of the electric charges. The space charges are accumulated in the insulation layer 3 in accordance with the time of using the cable, and when an impulse voltage is applied to the cable or polarity of the DC voltage applied to the cable is abruptly inverted, electric field strength around the conductor 1 is abruptly increased, and breakdown voltage of the power cable is lowered.
In order to solve the problem of DC dielectric strength and breakdown voltage of a cable, which are decreased due to the accumulation of space charges, a technique of adding an inorganic particle such as aluminum silicate, calcium silicate, calcium carbonate, magnesium oxide or the like to an insulation base resin configuring the insulation layer 3 is used in the prior art (see Japanese Patent Laid-Open Publication No. (S)57-21805 and Japanese Patent Registration No. 2541034 and 3430875). However, it is general that 1 to 80 parts by weight of the inorganic particle are added to the insulation layer 3 with respect to 100 parts by weight of base insulation resin and act as an impurity, and even when only a small content of the inorganic particle is added, impulse strength, which is another important feature required in the power cable, is lowered.
Meanwhile, a technique of using carbon nano-particles as an inorganic particle for suppressing accumulation of space charges in the insulation layer is disclosed (See Korean Patent Laid-Open Publication No. 2011-0138958). However, since the carbon nano-particles have a low dispersibility with respect to a base resin such as polyethylene, polypropylene or the like of hydrophobicity and form aggregates in the base resin, they cannot efficiently reduce accumulation of the space charges and thus may lower the impulse strength.
As described above, since the impulse strength is lowered when inorganic particles are added to suppress accumulation of space charges, thickness of the insulation layer 3 of a DC power cable is determined by the impulse strength of the cable, rather than the breakdown voltage, and since the outer diameter of the cable is increased accordingly, it is problematic from the manufacturing and economical viewpoint.
Accordingly, required is an improved insulation material for a DC power cable, which can accomplish two features required in the DC power cable, i.e., suppressing decrease of DC dielectric strength and breakdown voltage of the cable caused by accumulation of space charges and suppressing decrease of impulse strength without increasing the outer diameter of the cable.